1. Field of the Invention
The present invention relates to a device manufacturing method and a substrate for use in the method.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. The lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device may be used to generate a desired circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist).
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
Certain ICs in telecommunications applications comprise a structure known as “T-gates.” T-gates are typically required to have gate sizes less than 0.4 μm. GaAs FET performance depends on the gate width which is the short dimension of the gate structure. As the gate length is reduced, gate resistance increases due to the reduced cross-sectional gate area. This increased resistance adversely effects device performance.
Therefore, to decrease the gate resistance and maintain a small gate length, additional gate material is added to the top of the gate feature thereby creating the shape of a “T”. This increases the total cross-sectional area without increasing the short dimension of the gate structure. For this reason, T-gates are also sometimes referred to as “mushroom gates”. Typical dimensions of a T-gate are a bottom dimension of 0.15–0.25 μm and a top dimension of 0.45–0.75 μm.
Presently T-gates are expensive to manufacture. T-gates can be made by an electron beam process in which a substrate 1 is coated with three layers of polymethylmethacrylate which have different sensitivities to the electron beam radiation, as illustrated in FIG. 2a. The middle layer 102 is the most sensitive, followed by the top layer 103 and then the bottom layer 101 which is closest to the substrate.
When the electron beam PB impinges on these layers (FIG. 2b), and following developing, the middle layer looses the largest portion 120, the top layer looses a slightly smaller portion 130 and thereby overhangs the gap in the middle layer whilst the bottom layer looses the smallest portion 110 as shown in FIG. 2c. Then by depositing a metal layer 140 on the substrate, a T-gate shape 50 is formed (FIG. 2d). Because the gap 120 in the middle layer is larger than the gap 130 in the top layer, there is a gap between the second layer 102 and the deposited metal 140 at the same level as the second layer 102. This allows the removal of the three electron beam radiation sensitive layers by dissolving in a solvent (so called “lift-off”).
Unfortunately, electron beam lithography is expensive and, because of the need to write the image, electron beam lithography generally has a low throughput.